docs: Adds guides for plugins and storefront starters (#840)

docs/content/troubleshooting/missing-payment-providers.md

@@ -0,0 +1,29 @@
+# Stripe not showing in checkout
+
+You add payment providers to your Medusa instance by adding them as plugins in `medusa-config.js`:
+```jsx
+const plugins = [
+  ...
+  // You can create a Stripe account via: https://stripe.com
+  {
+    resolve: `medusa-payment-stripe`,
+    options: {
+      api_key: STRIPE_API_KEY,
+      webhook_secret: STRIPE_WEBHOOK_SECRET,
+    },
+  },
+  ...
+];
+```
+And installing them through your favourite package manager:
+```bash
+yarn add medusa-payment-stripe
+# or
+npm install medusa-payment-stripe
+```
+
+Though, to be able to also show them as part of your checkout flow, you need to add them to your regions. 
+
+In Medusa Admin go to Settings -> Regions and select your newly added payment provider:
+
+![](https://i.imgur.com/CfR9BCV.png)